This invention relates to a device for preparing a chemically reacting mixture and for ejecting the mixture into a mold cavity, the mixture comprising at least two components, in particular a mixture of isocyanate and a polyol that reacts completely to form polyurethane. The device comprises a mixing head having an outwardly open bore, and a reciprocable ram coupled to the head, the ram having an ejector rod axially movable in the bore for therewith defining a mixing chamber and for ejecting the mixture from the chamber.
West German Patent No. 2065841 discloses a device of this general type for preparing molded bodies of polyurethane. The mixing head of this device has a bore with an open face which forms the exit opening for the mixture, and to which the cavity of a mold is connected to prepare a molded body of polyurethane. Opposed infeed nozzles are located between opposite ends of the bore in the mixing head for respectively injecting isocyanate and a polyol into the bore. Mixing results from the impact of one chemical on the other. The bore defines a mixing chamber together with an axially movable rod of a reciprocable ram. The rod moves axially in the bore for ejecting the mixture from the chamber. The ram is known as a cleaning ram, and while the components are being mixed in the chamber, the ejector rod of the ram is located above the opposing inlet nozzles.
The reaction which forms polyurethane has a very high adhesion strength such that the residues of the mixture tend to cling to the rod of the cleaning ram. Thus, extremely high hydraulic pressures are required to axially shift the rod during operation of the device. As a result, the rod of the cleaning ram tends to frequently break during repeated and frequent use.